In present day data processing systems, it is desired to provide a large amount of memory which can be accessed in a minimum amount of time. One type of memory which has enjoyed widespread use in the data processing field is that of magnetic media disk memories.
In general, disk memories are characterized by the use of one or more magnetic media disks stacked on a spindle assembly and rotating at a high rate of speed. Each disk is divided into a plurality of concentric "tracks" with each track being an addressable area of the memory array. The individual tracks are accessed through magnetic "heads" which fly over the disks on a thin layer of air. Typically, the disks are two-sided with a head accessing each side.
The heads are in substantial alignment and are mounted to an actuator motor which moves the heads from track to track during the reading and writing of information from the disks. The actuator motor may be a "voice coil" electrodynamic motor which has a coil moving within a permanent magnetic field, defining a cylindrical core.
Information is encoded on magnetic media disks as a series of binary bits indicating a "1" or a "0". These bits are encoded as the presence or absence of a magnetic flux reversal. The capacity of the storage disk is dependent on the number of flux reversals which can be accurately written onto and read from a magnetic media storage disk.
In present day technology, the magnetic flux reversals are written onto, and read from, the magnetic media through the use of thin film heads. As the magnetic head passes over the disk surface, the head differentiates the flux emanating from the media producing a series of Lorentzian pulses having alternating polarity. These isolated pulses are coupled to a read data channel which recovers the digital information recorded on the media.
In most conventional digital magnetic recording systems the readback (or playback) signal is differentiated in order to convert the waveform amplitude peaks into zero-crossings. An example of a read channel which uses peak detection as a means to recover data is described in section 2.4 of "Magnetic Recording Volume II: Computer Data Storage" authored by C. Dennis Mee and Eric D. Daniel. FIG. 2.39 of Mee and Daniel illustrates a differentiator-based peak detection data channel in block diagram form. In the channel of Mee and Daniel, the readback signal received from the head preamplifier is first equalized in order to achieve higher bit densities. A typical equalization filter uses pulse slimming filters to narrow both the leading and the trailing edges of the Lorentzian input pulse. Pulse slimming is generally achieved through the addition and subtraction of signal-derived compensation pulses. Following equalization, the readback signal is differentiated, limited and then inverted in order to fully recover the representative digital data.
Because of the demand for still higher bit densities, it is desirable to further narrow the time window (which defines a data bit) of signals read by the magnetic head so that more signals may be read in a given time and correspondingly greater information density may be achieved. This requires a substantial improvement in the read data channel electronics. Derivative and pulse/slimming equalizers generally attempt to narrow the time window by symmetrically narrowing or slimming the pulses. Pulse slimming, however, results in an increase in the bandwidth of the read pulses requiring a corresponding increase in the read system bandwidth. A larger bandwidth obviously introduces more noise into the system; consequently, the signal-to-noise ratio (SNR) of the channel is degraded. An increase in system noise frequently translates into large amounts of peak shift due to intersymbol interference. Peak shift causes significant reduction in the achievable window margin in magnetic media storage systems.
Therefore, it is an object of the present invention to provide a read channel detector which makes efficient use of bandwidth to achieve a high data rate (capacity) at an acceptably small bit-error rate (BER).
It is another object of the present invention to provide a read channel which increases the binary signalling speed capability without severe intersymbol interference and without degradation of SNR.
It is yet another object of the present invention to provide a read channel detector which results in improved recording performance in terms of added SNR margin, increased lineal density, reduced track width and narrower time window margins.
As will be seen, the present invention provides a more efficient means of signal detection, while approximately doubling the recording system's speed capability as compared to prior art read channel detectors. The preferred embodiment of the present invention has a 2:1 bandwidth reduction over conventional differentiator detectors.